This invention relates to a rocker arm manufactured by pressing which is rocked by a cam to open and close the valve of a cylinder head (hereinafter referred to merely as "a rocker arm", when applicable), and a method of manufacturing the rocker arm, and more particularly to an improvement for the rocker arm such that a rocker arm body has so-called "two-piece structure".
A rocker arm of so-called "one-piece structure" is known in the art in which three parts, namely, roller supporting side walls, a pivot engaging portion, and a valve engaging portion are formed as one unit by using one piece of metal plate, as disclosed by U.S. Pat. No. 5,016,582.
The known rocker arm is formed as follows: First, one piece of metal plate is blanked to obtain a base material which is substantially rectangular and has an opening at the center. The base material is bent and drawn on the press, to form the desired rocker arm. The valve engaging portion of the rocker arm is curved distinctively. Hence, it is difficult to shape the valve engaging portion as required, and even if the valve engaging portions are formed, they are not uniform in configuration. Therefore, there has been proposed a rocker arm having a so-called "two-piece structure" in which the valve engaging portion is separated from the remaining parts.
The term "two-piece structure" as used herein is intended to means the structure which includes: a first piece comprising the roller supposing side walls and the pivot engaging portion; and a second piece which is the valve engaging portion.
The rocker arm having a "two-piece" structure has, however, the following disadvantages: In joining the second piece, namely the valve engaging portion, to the first piece, there is considerable difficulty in determining the position of the valve engaging portion. Hence, when the rocker arms are manufactured, they are not uniform with respect to the relative positions of the roller, the pivot engaging portion, and the valve engaging portion. The two piece rocker arms have a low manufacturing yield because the valve engaging portion, being distinctively curved, has no flat portion at all, and in joining the valve engaging portion to the first piece, it is difficult to obtain a reference point for positioning the valve engaging portion. Likewise, the pivot engaging portions of one piece rocker arms are very difficult to form since the side walls are integral with both the pivot engaging portion and the valve engaging portion. Side walls on the one piece rocker arms restrict cold forming operations on both the pivot engaging portion and the valve engaging portion.
These limitations on existing one piece rocker arms become particularly significant when attempting to form a rocker arm for use with an overhead cam engine having inclined lifter posts. The continuous structure of the side walls to the pivot engagement portion makes it very difficult to cold form the pivot engagement portion or the semispherical cavity of the pivot engagement portion to an incline as required for engagement with an inclined lifter post. The side walls restrict such a cold forming operation on the pivot engagement portion, and furthermore, such a cold forming operation on a one piece rocker arm would adversely affect the accuracy of the position of the portion of the rocker arm that engages with the valve stem in relation to the cam engaging roller. An additional disadvantage of cold forming a pivot engagement portion of the rocker arm that is integral with the rest of the rocker arm as disclosed by Mills, is that any finishing work such as grinding of the semispherical cavity of the pivot engagement portion must be carried out while the rocker arm main body together with the pivot engagement portion are held stationary in a special holding jig.